Capnography Lecture Review

Capnography Overview

  • Capnography detects cessation of pulmonary blood flow (e.g., pulmonary embolism, cardiac arrest).
  • Continuous quantitative waveform capnography is recommended for ET placement confirmation during CPR.

Capnography Basics

  • Measures CO2 concentrations in expired gas using infrared spectroscopy.
  • Capnography is continuous; capnometry is numerical without a waveform.
  • End-tidal carbon dioxide (ETCO2) is measured at the end of exhalation, reflects alveolar CO2.
  • ETCO2 is roughly equal to arterial CO2 under normal ventilation/perfusion; indicates respiratory cycle changes.

Capnography Types

  • Chemical Measurement: Disposable colorimetric detectors provide qualitative estimates, yellow indicates proper placement, purple indicates poor placement.
  • Mainstream vs. Sidestream: Mainstream is faster, attaches directly to ET; sidestream involves sampling away from the airway, resulting in more delay.

Capnography Monitoring

  • Standard for moderate and deep sedation as of July 1, 2011, per American Society of Anesthesiologists.
  • Continuous ETCO2 monitoring can signal early hypoxia and detect respiratory depression ahead of oxygen saturation changes.

Interpretation of Waveforms

  • Flat waveforms indicate apnea; inspired air has negligible CO2; expired air has 4.5%-5.5% CO2.
  • Waveform stages:
    1. Phase 1: Initial exhalation from conducting airways with low CO2.
    2. Phase 2: Alveolar gas mixes, CO2 concentration rises.
    3. Phase 3: Alveolar plateau, PetCO2 reached.
    4. Phase 4: Inspiration, CO2 concentration drops to zero.

Physiological Considerations

  • PetCO2 depends on PACO2, influenced by CO2 production (VCO2) and ventilation effectiveness. Conditions can increase/decrease metabolic rate and CO2 production.
  • ETCO2 normally 4-6 mm Hg lower than PaCO2 due to ventilation/perfusion mismatch.

Capnographic Contours

  • Changes in capnogram can indicate dead space ventilation, hyperventilation, hypoventilation, apnea, and ineffective neuromuscular blockade.

Clinical Applications

  • Monitor effectiveness of gas exchange in CPR; recognize ventilatory issues such as hypoventilation and hyperventilation.
  • Specific patterns (e.g., shark fin pattern for obstruction) can inform on severity of respiratory issues.